Continuously variable transmissions (CVTs) are commonly used on a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts, scooters, etc. They typically comprise a driving pulley mechanically connected to a motor, a driven pulley mechanically connected to wheels or a track, possibly through another mechanical device such as a gear box, and a trapezoidal drivebelt transmitting torque between the driving pulley and the driven pulley. A CVT automatically changes the ratio as required by load and speed conditions, providing an increased torque under high loads at low speeds and yet controlling the rotation speed of the motor as the vehicle accelerates. A CVT may be used with all kinds of motors, such as internal combustion engines or electric motors.
The sides of the drivebelt are, on each pulley, gripped between two opposite sheaves that are coaxially mounted around a corresponding main shaft. Generally, in each pulley of a conventional CVT, one sheave, usually called “fixed sheave”, is rigidly connected to one end of the corresponding main shaft. The other sheave, usually called “movable sheave”, is free to slide and/or rotate with reference to the fixed sheave by means of bushings or the like.
At a low vehicle speed, the winding diameter of the drivebelt at the driving pulley is minimal and the winding diameter of the driven pulley is maximum. This is referred to as the minimum ratio since there is the minimum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
Generally, when the rotation speed of the driving pulley increases, its movable sheave moves closer to the fixed sheave thereof under the effect of a centrifugal mechanism. This forces the drivebelt to wind on a larger diameter on the driving pulley and, consequently, on a smaller diameter on the driven pulley. The drivebelt then exerts a radial force on the sheaves of the driven pulley in addition to the tangential driving force by which the torque is transmitted. This radial force urges the movable sheave of the driven pulley away from the fixed sheave thereof. It is counterbalanced in part by a return force, which is typically generated by a spring inside the driven pulley or another biasing mechanism. It is also counterbalanced by a force generated by the axial reaction of the torque applied by the drivebelt on the driven pulley. This is caused by a cam system that tends to move the movable sheave towards the fixed sheave as the torque increases. The cam system typically comprises a cam plate having a plurality of symmetrically-disposed and inclined ramps on which respective cam followers are engaged. The followers are usually sliding buttons or rollers. The set of ramps or the set of followers is mounted on the movable sheave and the other is directly or indirectly connected to the main shaft in a rigid manner. The closing effect of the cam system on the drivebelt tension is then somewhat proportional to output torque.
Generally, at the maximum vehicle speed, the ratio is maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley. Then, when the vehicle speed decreases, the rotation speed of the driving pulley typically decreases as well since the rotation speed of the motor decreases. This causes, at some point, a decrease of the winding diameter of the driving pulley and a decrease of the radial force exerted by the drivebelt on the sides of the sheaves at the driven pulley. Ultimately, the driven pulley is allowed to have a larger winding diameter as the spring or another biasing mechanism moves the movable sheave back towards the fixed sheave.
Some CVTs are provided with reversible driven pulleys. A reversible driven pulley operates in a similar fashion than that of a conventional one, with the exception that the transmission ratio can be controlled during motor braking or when the vehicle is traveling in reverse. For instance, during motor braking, the torque is no longer coming from the motor to the wheels or track, but in the opposite direction. Similarly, when accelerating in reverse, the torque and the rotation will be in the reverse direction, the torque being transmitted from the motor to the wheels or track. A reversible driven pulley generally comprises a second set of ramps and a second set of followers. In use, one set of followers and its corresponding set of ramps are used when the torque is in one direction, the other set being used for the other direction.
U.S. Pat. No. 6,949,039 shows an example of a driven pulley. The driven pulley described therein provides many advantages in terms of overall weight reduction and compactness. Nevertheless, there is still room for further improvements, including some to lower the manufacturing costs thereof.